The cloud layer of stratus is always very low. If it is divided into ragged masses in a wind or by mountain tops, it may be called fracto-stratus. The complete absence of detail of structure differentiates stratus from other aggregated forms of cloud.
We have given the foregoing official definitions and descriptions in full in order to aid the reader as much as possible, so far as verbal information goes, in learning to call the common clouds by their names. Good pictures are, of course, an essential part of this process, and apart from those that illustrate the present text, many collections of such pictures are easy of access. Some may be obtained free or at nominal cost from the Weather Bureau in Washington and from the Meteorological Office in London. The “International Cloud Atlas” (second edition, Paris, 1910) is now out of print, but may be consulted in libraries.
Of the clouds above enumerated, cirrus, cirro-cumulus, and cirro-stratus are the highest, and are always ice clouds. They consist in some cases of separate, minute crystals—a fine dust of ice—producing, according to the forms of the crystals, one or another of the various forms of halo around the sun and moon; while in other cases the crystals are aggregated in small snowflakes, so that the cloud is a real snowstorm in midair. The altitude of these clouds generally ranges from 4 to 8 miles. In the equatorial region their height is often 10 miles or more. The other main types of cloud are composed wholly or chiefly of water. Alto-cumulus and alto-stratus are clouds of medium altitude; strato-cumulus and nimbus are low clouds (generally not more than a mile high); while stratus, the lowest cloud of all, grades into fog, which commonly rests on the earth. Since cumulus and cumulo-nimbus are produced by the condensation of moisture from rising air currents, the height of their bases varies widely with the temperature and humidity of the lower air; the average height is rather less than a mile. Their vertical extent, however, is much greater than that of the other cloud types. Cumulo-nimbus sometimes towers to a height of 4 or 5 miles above its base, and it is then commonly crowned with ice clouds, including a filmy “scarf cloud” draping the summit, and spreading wisps of so-called “false cirrus,” drawn out horizontally by the upper winds.
Besides the ten main classes of clouds, a few distinct minor varieties are recognized by all meteorologists. Among these is the “lenticular cloud”; an isolated small cloud, which frequently shows iridescence, and the shape of which has been compared to that of a lens or an almond. This cloud may remain stationary, or nearly so, but it really marks the position of a billow in a stream of air, the moisture condensing at one edge of the cloud and dissolving at the other. Another distinctive and rather rare form of cloud, seen chiefly in connection with thunderstorms, is mammato-cumulus, likewise known as “pocky cloud,” “festoon cloud,” “rain balls,” etc. It consists of numerous sacklike or udderlike protuberances, convex downward.
When a stream of moist air is forced to ascend in passing over a mountain its moisture is often condensed by the process of dynamic cooling, already explained, and a “cloud cap” is seen over the summit. In local weather lore such caps are generally regarded as a sign of rain. These clouds attached to mountains were called “parasitic clouds,” by writers of a century ago, who proposed some naïve explanations of them. Occasionally a “cloud banner” streams far to the leeward of the mountain. One of the most famous and striking of cloud caps is the “tablecloth” that spreads over Table Mountain, near Cape Town, when a moist wind blows in from the sea. Sometimes the local topography causes the wind that has swept up over a mountain to form a second “standing” wave to the leeward of the summit, and this may also be marked by a cloud, which, like the cloud cap, presents a delusive appearance of permanence, while it is, in reality, in constant process of formation on the windward side and dissipation on the leeward. The two clouds thus formed, one over the summit and the other to the leeward, are often seen at Table Mountain, and are further exemplified in the celebrated “helm and bar” of Crossfell, in the English Lake District.
In the case of a wind blowing athwart a ridge or mountain range, a bank of cloud may extend along the whole crest, as in the “foehn wall” that appears along Alpine heights when the foehn wind is blowing.
Some day meteorology will be taught in art schools, for the same reason that anatomy now is. When that blissful day arrives painters will probably show us skies less at odds with nature than those that deface the work of artists of all degrees of celebrity, including the “old masters.”